Technical Field
The present invention relates generally to the control of noise in communication channels. More specifically, the present invention relates to the control of noise in communication channels that use broadband chirps.
Description of Related Art
Broadband chirp signals have been found useful in various communication scenarios. One particularly challenging scenario relates to acoustic communications “downhole” in the field of drill-based hydrocarbon exploration and extraction.
Downhole conditions are hostile with unstable, difficult communication conditions and high temperatures. The high temperature environment, such as a deep well, restricts hardware computational resources to low speed processors with small amounts of on-board memory. Unstable communication channels (whether electromagnetic, acoustic or wired) or the need to prioritize data processing leads to the requirement for flexibility in managing resources as conditions change.
One of the roots of instability in such communication channels is the present of noise. It is for this reason that chirp pulses are attractive—they are relatively immune to noise. For a given scenario certain frequencies can be expected to be more prone to noise than others. It is therefore desirable that the communication channel used for downhole communications is dynamically selected to correspond to frequencies least exposed to the effects of noise. Where noise is present, it is also desirable that the communication channel does not overcompensate by requiring excessive transmission power levels.
“In well” communication by means of acoustic pulse transmission and reception (i.e. acoustic telemetry) along a drillstring is severely limited by the dynamic and non-stationary nature of the channel both in terms of noise and channel transfer function. One main issue in downhole communications is to ensure that signals sent by a first unit (i.e. transceiver node) in an acoustic telemetry system reach and are detected by a second unit in that system, even when the transmission lies well inside noise (i.e. having an amplitude comparable to noise in the transmission medium). The receiving unit must then be able to remove any noise and recover the signal.
The presence of noise in current acoustic telemetry systems means that relays are required to boost the signal beyond a maximum of 2,500 m in a vertical deployment and 950 m in a horizontal deployment.
It is known to use chirp codes (i.e. linear frequency modulation) in signal propagation. A chirp code offers a high signal to noise ratio and therefore allows transmission through high noise environments. A typical chirp pulse is a frequency sweep pulse with a short autocorrelation function.
Chirp pulses may be any pressure wave signal capable of pulse compression. Chirp pulses have the property that the longer the pulse length (often derived from the time-bandwidth, TB, product) the better the immunity to noise without loss of resolution. The relationship is given by the following equation:S/N=10·log(SQRT(TB))  (eq. 1)
This is illustrated in FIG. 6.
The drillstring may be considered as a series of (steel) pipes of uniform length connected by short couplings having different physical properties: this can effectively limit practical communications to a number of “passbands”. The issue is discussed in greater detail in an article entitled “Wave impedances of drillstrings and other periodic media”, Drumheller. Douglas S, (Journal of the Acoustical Society of America, Volume 112, Issue 6, pp. 2527-2539 (2002)).
Typically passbands having the lowest frequencies are considered to contain unacceptably high levels of noise and are therefore thought not to be viable for the purpose of downhole communications. In the case of chirp pulses, the longer the pulse length the better the immunity to noise. This opens the possibility of using passbands that were previously avoided.
To achieve an increased chirp length (and thus increased immunity to noise without loss of resolution), known systems adopt lower bit-rates. They also rely on using a narrow section of the passbands in order to ensure that any variation in width or position of the band is compensated for. In addition, these known systems rarely use more than one passband, again limiting the performance and flexibility of the device.
In summary, current systems have limited their technical solution in order to gain reliability but have sacrificed performance, flexibility, and (ironically) reliability.
Known techniques do little to address noise immunity and typically require pumping/drilling to stop—low frequency passbands are avoided due to high intrinsic background noise.